Thin film adjustable attenuator with s-shaped bifurcated contacts



DEC. 29, 1970 T EMMQE'T ET AL 55fi,@?2

THIN FILM ADJUSTABLE ATTENUATOR WITH S-SHAPED BIFURGATED CONTACTS 5 Sheets-Sheet 1 Filed Fgb. 20, 1968 J. T.EMMOTT m/va/vrons g, LESH M. EV 8V ATTORNEY Dec. 29, 1970 11 EMMOTT ET @fifiifi??? THIN FILM ADJUST E ATTE T0 ITH S-SHAPED BIF ATED c ACTS Filed Feb. 20, 1968 3 Sheets-Sheet 2 FIG. 3a

BONDED LEAD J. T. EMMOTT ET AL 3,i,@?2 THIN FILM ADJUSTABLE ATTENUATOR WITH S-SHAPED Dec. 29, 1970 BIFURCATED CONTACTS Filed Feb. 20, 1968 3 Sheets-Sheet 5 United States Patent O 3,551,872 THIN FILM ADJUSTABLE ATTENUATOR WITH S-SHAPED BIFURCATED CONTACTS John T. Emmott, East Hanover Township, Morris County, Nathan G. Lesh, Livingston, and Mead F. Stevens, Pompton' Plains, N.J., assignors to Bell Telephone Laboratories, Incorporated, Murray Hill and Berkeley Heights, N .J., a corporation of New York Filed Dec. 20, 1968, Ser. No. 785,715 Int. Cl. H01c 1/12, 9/02 US. Cl. 338-202 3 Claims ABSTRACT OF THE DISCLOSURE An adjustable attentuator is provided by an integral unit which includes a thin film substrate with a preselected pattern of area resistors deposited thereon. Dual spring contacts, arranged for sliding movement across areas of the film, selectively bridge different portions of the resistive pattern to provide incremental adjustments in the attenuation level.

BACKGROUND OF INVENTION (1) Field of the invention This invention relates to adjustable attenuators and more particularly to attenuators employing thin film resistive patterns.

(2) Description of the prior art The employment of thin film deposits on non-conductive substrates to form miniature resistive attenuators is known in the art, being shown, for example by H. F. Kinkel et al. in US. Pat. 3,344,387, issued Sept. 26, 1967. Kinkels device utilizes a flat nonconductive base or substrate with a plurality of discrete resistive film areas deposited thereon. A plurality of contacts are contiguous with and located along each side of each of the resistive film areas. An insulating cover member with printed circuit paths on the inside surface thereof is fitted over the base portion to form the completed component. To provide connecting points, the base includes three conductive land areas deposited at one end portion thereof with a terminal extending outwardly from each of the land areas. Corresponding land areas on the inside of the cover are placed to ensure registry with the land areas on the substrate.

Similarly, contact buttons on the inside cover are placed to ensure registry with the contacts on the substrate. By selection of any two of the plurality of contacts on the resistance paths of the substrate, a desired length of film can be employed to provide a preselected resistance. The cover, as initially formed, includes sufiicient circuit paths to interconnect with the paths on the base in several different patterns, each resulting in a different resistance. The unwanted paths are then removed from the cover selectively in accordance with the particular resistance pattern desired. Changing the resistance of the attenuator thus requires changing the resistive pattern on the cover or, alternatively, a plurality of covers may be employed for a single base and a resistance change may be effected by changing the cover.

Thin film attenuators of the type described although adequate for some circuit needs, are obviously unsuitable for circuit environments that require frequent attenuation changes. Accordingly, an object of the invention is to simplify the adjustability aspect of adjustable thin film attenuators. Another object is to simplfy the overall construction of thin film attenuators and thereby reduce the cost thereof.

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SUMMARY OF THE INVENTION The stated objects and related objects are achieved in accordance with the principles of the invention by an adjustable attenuator employing a pattern of thin film area resistors and connecting conductive areas deposited on a common substrate in combination with spring loaded contacts which are slidably adjustable across the surface of the thin film deposits. The configuration of the contacts permits selective bridging of discrete areas of the thin film and the level of attenuation is thus readily attained by the simple movement of a sliding switch. The high degree of miniaturization achieved by unique layer and surface pattern configurations of area resistors allows a plurality of attenuation adjustments to be derived from a single substrate base in combination with a like plurality of sliding switches.

In one embodiment of the invention a complete multiple adjustment attenuator arrangement is packaged in an assembly measuring approximately 1 /2 X 1" x A". Each of four conveniently positioned slide switches on one surface of the assembly may be moved from a neutral to a full throw position to introduce progressively greater magnitudes of attenuation. The opposite side of the assembly casing includes an integral terminal mounting arrangement and each of the terminals is mounted over a corresponding aperture in the casing to facilitate lead connections from related thin film areas.

In accordance with one feature of the invention, the thin film resistive elements of the attenuator are combined into several balanced H pads deposited on a common substrate. The element values are adjusted, as by anodizing for example, to meet the combined performance parameters of insertion loss and impedance rather than absolute resistance. As a result, maximum yield is realized during production which obtains from the fact that insertion loss is virtually independent of impedance, i.e., the geometry determines the insertion loss and hence the resistance ratio exclusive of impedance. Impedance is dependent only on the resistivity and has relatively broad tolerances of two to three percent, for example, while the tolerance of insertion loss may be as close as 0.1 percent. Accordingly, when anodizing, it is possible to accept the relatively broad limits of plus or minus three percent on resistivity by selectively anodizing to the required voltage ratio.

In accordance with another feature of the invention, the switch contacts, as noted above, perform their function by sliding across the surface of the thin film substrate and the termination area of the thin film circuit is utilized as one contact of each slide switch. It is this feature which contributes to the overall compactness and simplicity of an attenuator in accordance with the invention.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of an assembled multiadjustment attentuator in according with the invention;

FIG. 2 is an exploded view drawn to an enlarged scale of the attenuator shown in FIG. 1;

FIG. 3 is a partially exploded view of the underside of the attenuator shown in FIG. 2;

FIG. 4 is a perspective view of the thin film circuit shown in FIG. 2 drawn to a distorted scale to illustrate the layer configuration; and

FIG. 5 is a plan view drawn to an enlarged scale of the thin film circuit shown in FIG. 2.

DETAILED DESCRIPTION The individual elements of the attenuator 101 of FIG. 1 may readily be identified from the exploded view shown in FIG. 2. The switch housing includes a lower housing member or base 201 interfitted with an upper housing member or cover 205. The thin film resistive circuit 202 rests within the frame formed by the base 201. Apertures 208 in the base 201 are provided to accommodate terminating leads, not shown, which are aflixed to the terminals 501 through 504 on the thin film circuit shown in FIG. 5. These leads are threaded through the apertures 208, also shown in FIG. 3, and through the corresponding apertures 302 of the connecting pins 301 and are suitably affixed thereto as by soldering, for example. Each of the connecting pins 301 includes a respective one of the output terminals 207. Each of four slide bars 203 is housed for sliding movement within the cover 205 and is in alignment with a respective one of the slots 209. Each of the bars 203 includes a raised aperture or shoulder portion 212 which accommodates a respective one of the downwardly projecting studs 210 of a corresponding one of the thumb buttons 206. By means of a simple snap fit, each of eight spring contactors 204 is affixed to and is supported by a respective end portion of one of the slide bars 203. As shown, each of the spring contactors 204 is placed so that the dual contact elements 211 extend downwardly to permit engaging relation with the surface of the thin film element 202.

In assembling an attenuator in accordance with the invention, the cover 205 may advantageously be joined to the base 201 and the buttons 206 may be afiixed to the slide bars 203 by means of ultrasonic bonding. This is wellknown technique is simple and effective and is readily adapted to mass production procedures. Heat treating after bonding is desirable to stress relieve the molded plastic piece parts which may advantageously be formed from Lexan, a commercially available polycarbonate.

FIG. 3 shows that the underside of the base 201 is provided with a series of bosses or raised areas 303 which include slots 304 for accommodating or positioning the shank portion of each of the terminal pins 301, so that the apertures 302 are in correspondence with the apertures 208 and so that the connecting terminals 207 are positioned in the manner shown in FIG. 1. A terminal indicating plate 305 which is bonded to the tops of the bosses 303 secures the terminals 301 in place.

One illustrative configuration for the thin film circuitry which incorporates the resistive H pads is shown in detail plan view in FIG. 5, and a portion thereof is shown in FIG. 4 to illustrate typical layer compositions and layer thicknesses. The small rectangles 50A, B through 61A, B on the thin film surface mark slightly raised land areas that are in conductive relation with the contacts 211 of the spring contactors 204. As shown in FIG. 4 these areas are formed from layers of gold, platinum or rhodium and gold. Any one of a number of materials such as glass, glass wth an oxide underlay, glazed alumina or superfine alumina may be used to form the substrate for the thin film circuits.

A better appreciation of the fact that the resistive thin film patterns shown on FIG. 5 are the equivalents of conventional lumped element resistive H pads may be gained by examining the correspondence between the terminals of an equivalent lumped element H pad and the area type terminals of one of the FIG. 5 deposited H pads. A conventional H pad has four external terminals corresponding to the two termination points on each of the two exterior leg or verticals. Typically, there is a pair of resistors in each of the two legs, the external terminals of the pad corresponding to the unconnected terminals of these resistors and the common terminals of the resistors forming the internal terminals of the cross bar or horizontal member which normally includes still another resistor. In FIG. 5 area terminals 55A and 55B correspond to the external terminals of one vertical leg of the H, and terminals 72B and 54B correspond to the external terminals of the other leg of the H. When the two sets of bifurcated contacts mounted on the single slide which 4 controls the H pad just described are in the no-attenuation position, one set of these contacts shorts or connects together area terminals 55A and 55B, while the second pair of contacts connects or shorts the terminals 72B and 54B. The resistive areas on each of the two legs, shown in stippling, corresponding to the lumped resistive elements in a conventional H pad, are thus shorted out and are in effect removed from the circuit. When the slide is pushed to the position of attenuation, however, one set of contacts connects terminals A and 60B, and the other set of contacts is shifted to a neutral position across the terminals 54A and 54B. This position is termed neutral in view of the fact that these terminals are contiguously conductive and no change in the circuit is introduced by bridging across such terminals. The terminals 60A and 60B correspond to a pair of terminals which require connection in order to complete a circuit path across the cross bar or horizontal member of the H pad. Without the area terminals 60A and 60B connected, there is an open in the circuit which of course corresponds to the noattenuation position. With these terminals connected, however, the H pad circuit is completed between the internal terminals and over the cross bar and the external terminals of the vertical legs remain unshorted, Similar correspondence between each of the three additional resistive thin film or area H pads and a conventional lumped element H pad may be traced in the manner indicated.

Attenuators in accordance with the invention may be employed to particular advantage in various communication equipment where the maintenance of precise levels of attenuation are critical to effective operation. Such equipment may, for example, include inband signaling units and high frequency repeater units. In one illustrative system, two attenuators of the type disclosed herein, each employing four slide bar units, are utilized to cover a 16.5 db range of attenuation. The units differ only in the substrate pattern, although color coding may advantageously be employed to ensure proper identification. When the two units are connected in series, the two substrate patterns provide loss increments of 0.1, 0.2, 0.4, 0.8 and 1, 2, 4, 8 db respectively at constant impedance. Each of the eight two-position slide switches can be moved manually from a zero loss position to one of the abovelisted increments.

It is to be understood that the embodiment described herein is merely illustrative of the principles of the invention. Various modifications thereto may be etfected by persons skilled in the art without departing from the spirit or scope of the invention.

What is claimed is:

1. An adjustable attenuator comprising, in combination, a plurality of thin film resistive elements deposited on a common substrate, a base member including support means for said substrate, a cover member positioned over said base member thereby to form an attenuator housing, a plurality of contact carrying slide members each in combination with a manually movable button member aligned with a corresponding cover slot thereby to permit slidably adjusting the position of said slide members with respect to said thin film resistive elements, first and second S-shaped spring contact members mounted on each of said slide members, the top portion of each of said S-shaped contact members being spring fitted around a corresponding end of an associated one of said slide members, the bottom portion of each of said S-shaped contact members depending downwardly to and in predetermined contact with selected ones of said resistive elements, each of said bottom portions including bifurcated contacts whereby the movement of a single one of said slide members may be utilized to bridge two pairs of terminal points on said thin film resistive elements.

2. Apparatus in accordance with claim 1 wherein said resistive elements are formed into a plurality of H pads each corresponding to an associated one of said contact carrying slide members, each pair of said bifurcated contacts on one pair of said spring contact members being employed to short circuit a corresponding one of the outer legs on one of said H pads with the associated one of said slide members being in a first or no-attenuation position and one of said last named pairs of contacts completing the circuit for the internal cross leg of said last named H pad and the other of said last named pairs of contacts being placed in a neutral position whenever said last named slide member is in a second or full-attenuation position.

3. Apparatus in accordance with claim 2 wherein said resistive elements have been preadjusted to meet combined preselected performance parameters of insertion loss and impedance rather than absolute resistance.

References Cited UNITED STATES PATENTS 11/1958 Bourns 338-199X 9/1967 Kinkel 338-314 11/1967 Wollf 338-128X 3/1968 Yungblut 33 8183 8/1968 Soulakis 338184X 9/1968 Van Bethuysen 338183 LEWIS H. MYERS, Primary Examiner G. P. TOLIN, Assistant Examiner US. Cl. X.R. 

